1. Field of the Invention
The invention relates to a method for conveying silicon granules in an encapsulated conveying channel.
2. Background Art
In the production of polycrystalline silicon granules in a fluidized bed reactor it is necessary, over the course of the process, to meter silicon material into the reactor at regular intervals or continuously and elsewhere to remove finished granules from the reactor.
Simple open-closed fittings such as slides, for example, are usually used for this purpose in production. In order to preclude contamination of the high-purity silicon, slides composed of silicon material are generally used in this case. What is disadvantageous, in this case, however, is the high degree of wear of the slides, resulting from the abrasiveness of silicon granules. For this reason, therefore, these fittings have to be replaced in a short cycle.
A further disadvantage of the slides is a tendency toward blockage. In this case, the respective apparatus has to be shut down in order to manually eliminate the blockage in the fitting. Moreover, only limited regulation of the metering and extraction amounts can be achieved by means of the simple open-closed control.
Open and closed conveying channels with a direct coupling-in by means of electric or pneumatic vibration motors are usually used in the art for conveying bulk materials.
In the production of silicon single crystals, the loading and the recharging of a melting crucible with high-purity silicon material have to be carried out under a vacuum atmosphere for purity reasons.
U.S. Pat. No. 5,462,010 discloses a device for continuously conveying granular polysilicon into the melting crucible. The arrangement comprises a complex delivery system comprising a large holding container, an intermediate container and the main container, which is seated on a vibration drive. The entire system is housed in a common vacuum chamber. What is disadvantageous about this arrangement is the vibration drive, which is controlled externally in a complicated manner and which fosters losses of tightness in the system owing to the required perforations for electrical supply lines and controls. As a result of the high frequencies required to move the silicon material, high abrasiveness comes into question. Since the high-purity silicon granules in that case come into contact with many different materials (measuring probes, etc.), increased contamination of the material is also a concern.
U.S. Pat. No. 6,609,870 discloses a method for conveying granular silicon by means of a pneumatic device. In that case, the system is fitted in a closed system under a vacuum. The aim of this system is also to remove dust particles from the granules by means of the air flow. What is disadvantageous about the system, however, is that it is an open system wherein it is necessary constantly to add a high-purity gas as transfer medium, which is then contaminated with dust during transport.
In the production of polysilicon granules in a fluidized bed reactor, the reactor with its addition and extraction lines constitutes a closed system, by contrast, which generally operates with a design pressure of up to 10 bar excess pressure. For this reason, known vibrating conveying channels with an electric or pneumatic vibration motor which couple in directly at the vibrating channel cannot be used.